In a case when a rear-PTO shaft is to be additionally provided for the purpose of enlarging the usefulness of a tractor in which only a mid-PTO shaft is provided as power take-off shaft as shown, for example, in JP,U No.61-153631, a clutch mechanism is usually provided for enabling to drive selectively both of the mid- and rear-PTO shafts or any one of these PTO shafts.
As clutch mechanism of this purpose, there are known two types of the clutch mechanism. One of them is fashioned as shown, for example, in JP,U No.1-167921 such that it comprises a clutch, for connecting and disconnecting transmission of power to the mid-PTO shaft, and another clutch for connecting and disconnecting transmission of power to the rear-PTO shaft. The other type is shown in, for example, U.S. Pat. Nos. 4,613,024 and 4,880,070 and is fashioned such that it comprises a single PTO-selecting clutch which is operable to control transmission of power to both of the mid- and rear-PTO shafts.
A clutch mechanism of the former type having two separate clutches requires a complicated control mechanism. That is, when it is designed to control the two clutches individually, separate two control mechanisms are required and, when it is designed to control both of the two clutches simultaneously, a complicated link mechanism is required.
On the other hand, although a clutch mechanism of the latter type having a single PTO-selecting clutch may be operated by means of a simple control mechanism, this clutch mechanism involves a problem that it is difficult to make the structure and parts of a power take-off transmission assembly common, for a cost-saving purpose, to both of the tractors having only a mid-PTO shaft and having a rear-PTO shaft in addition to a mid-PTO shaft. That is, as shown in each of the U.S. Patents referred to above, a PTO-selecting clutch according to the prior art is incorporated in an axiliary implement-driving power transmission path at a location considerably remote from the mid- and rear-PTO shafts and are connected to these two PTO shafts by means of separate two transmission paths each having a considerable length. A power take-off assembly of this structure including a PTO-selecting clutch is largely different in structure from the one to be employed in a tractor in which a rear-PTO shaft and PTO-selecting clutch are not provided and only a mid-PTO shaft is provided.